diff --git a/CHANGELOG.md b/CHANGELOG.md
index f8a9845..cc4dadc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,4 +44,3 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 [0.6.0]: https://github.com/jerry-git/pytest-split/compare/0.5.0...0.6.0
 [0.5.0]: https://github.com/jerry-git/pytest-split/compare/0.4.0...0.5.0
 [0.4.0]: https://github.com/jerry-git/pytest-split/tree/0.4.0
-
diff --git a/README.md b/README.md
index fd04e61..9936f21 100644
--- a/README.md
+++ b/README.md
@@ -39,6 +39,20 @@ The file path is configurable via `--durations-path` CLI option.
 pytest --store-durations
 ```
 
+With the `--store-durations` flag, you can also specify how test case runtime is managed:
+
+- `keep`: Only new test cases' runtime are added. The pre-existing durations in the .test_durations file are reused. This is useful when you've added new tests and want to keep the historical durations of older tests.
+
+```sh
+pytest --store-durations=keep
+```
+
+- `replace` (Default): All test case runtimes in the .test_durations file are replaced with the new ones from the current run.
+
+```sh
+pytest --store-durations=replace
+```
+
 Then we can have as many splits as we want:
 ```sh
 pytest --splits 3 --group 1
diff --git a/src/pytest_split/plugin.py b/src/pytest_split/plugin.py
index ef635f7..0d7bc70 100644
--- a/src/pytest_split/plugin.py
+++ b/src/pytest_split/plugin.py
@@ -33,8 +33,11 @@ def pytest_addoption(parser: "Parser") -> None:
     group.addoption(
         "--store-durations",
         dest="store_durations",
-        action="store_true",
         help="Store durations into '--durations-path'.",
+        type=str,
+        choices=["keep", "replace"],
+        const="replace",
+        nargs="?",
     )
     group.addoption(
         "--durations-path",
@@ -187,7 +190,7 @@ class PytestSplitCachePlugin(Base):
     The cache plugin writes durations to our durations file.
     """
 
-    def pytest_sessionfinish(self) -> None:
+    def pytest_sessionfinish(self) -> None:  # noqa: C901
         """
         Method is called by Pytest after the test-suite has run.
         https://github.com/pytest-dev/pytest/blob/main/src/_pytest/main.py#L308
@@ -216,6 +219,10 @@ def pytest_sessionfinish(self) -> None:
 
         if self.config.option.clean_durations:
             self.cached_durations = dict(test_durations)
+        elif self.config.option.store_durations == "keep":
+            for k, v in test_durations.items():
+                if k not in self.cached_durations:
+                    self.cached_durations[k] = v
         else:
             for k, v in test_durations.items():
                 self.cached_durations[k] = v
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 869c6a6..dfd5345 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -27,28 +27,66 @@ def durations_path(tmpdir):
 
 
 class TestStoreDurations:
-    def test_it_stores(self, example_suite, durations_path):
+    def test_it_stores_replace(self, example_suite, durations_path):
         example_suite.runpytest("--store-durations", "--durations-path", durations_path)
 
         with open(durations_path) as f:
             durations = json.load(f)
 
         assert list(durations.keys()) == [
-            "test_it_stores.py::test_1",
-            "test_it_stores.py::test_10",
-            "test_it_stores.py::test_2",
-            "test_it_stores.py::test_3",
-            "test_it_stores.py::test_4",
-            "test_it_stores.py::test_5",
-            "test_it_stores.py::test_6",
-            "test_it_stores.py::test_7",
-            "test_it_stores.py::test_8",
-            "test_it_stores.py::test_9",
+            "test_it_stores_replace.py::test_1",
+            "test_it_stores_replace.py::test_10",
+            "test_it_stores_replace.py::test_2",
+            "test_it_stores_replace.py::test_3",
+            "test_it_stores_replace.py::test_4",
+            "test_it_stores_replace.py::test_5",
+            "test_it_stores_replace.py::test_6",
+            "test_it_stores_replace.py::test_7",
+            "test_it_stores_replace.py::test_8",
+            "test_it_stores_replace.py::test_9",
         ]
 
         for duration in durations.values():
             assert isinstance(duration, float)
 
+    def test_it_stores_keep(self, example_suite, durations_path):
+        example_suite.runpytest("--store-durations", "--durations-path", durations_path)
+
+        with open(durations_path) as f:
+            durations = json.load(f)
+
+        default_keys = [
+            "test_it_stores_keep.py::test_1",
+            "test_it_stores_keep.py::test_10",
+            "test_it_stores_keep.py::test_2",
+            "test_it_stores_keep.py::test_3",
+            "test_it_stores_keep.py::test_4",
+            "test_it_stores_keep.py::test_5",
+            "test_it_stores_keep.py::test_6",
+            "test_it_stores_keep.py::test_7",
+            "test_it_stores_keep.py::test_8",
+            "test_it_stores_keep.py::test_9",
+        ]
+
+        assert list(durations.keys()) == default_keys
+
+        for duration in durations.values():
+            assert isinstance(duration, float)
+
+        example_suite.makepyfile("def test_11(): pass")
+        example_suite.runpytest(
+            "--store-durations", "keep", "--durations-path", durations_path
+        )
+
+        with open(durations_path) as f:
+            durations_keep = json.load(f)
+        assert list(durations_keep.keys()) == [
+            *default_keys,
+            "test_it_stores_keep0/test_it_stores_keep.py::test_11",
+        ]
+        for k in default_keys:
+            assert durations_keep[k] == durations[k]
+
     def test_it_overrides_existing_durations(self, example_suite, durations_path):
         existing_duration_test_name = "test_it_overrides_existing_durations0/test_it_overrides_existing_durations.py::test_1"
         old_value = 99